THIS INVENTION relates to a seal suitable for use in a volume displacement plethysmograph for investigating pulmonary function, particularly of infants. In particular, the invention is directed to a neck seal.
Objective assessment of respiratory function plays an important part in understanding the physiology and pathophysiology of the respiratory system. Lung function is an indispensable tool for diagnosis and monitoring of respiratory disease states in adults and older children, but has not gained wide acceptance in the management of infant lung disease. The primary difficulty in measuring infant lung function is the inherent lack of cooperation requiring assessment during sedated sleep and the use of a face mask. To date there is no widely applied technique for the measuring infant lung function in the unsedated infant.
Some methods of monitoring respiration in an infant have been developed for specific needs. Known devices for monitoring infant lung function can generally be classified as either invasive or non-invasive with respect to the infant""s airway.
Invasive monitoring devices include pneumotacchographs which are connected to a sealed face mask, or spirometers similarly connected to a face mask. Traditional methods of volume and flow measurement with a face mask and/or pneumotachograph induce error through the effects of trigeminal stimulation, increased dead space and resistive loading. Face masks are poorly tolerated in unsedated infants and induce arousal especially in light sleep. They are completely impractical in the awake infant.
In addition it is technically difficult to maintain a seal with the infant for protracted periods of time, thereby limiting the ability to acquire data dynamically during unpredictable respiratory events such as apnoeas, sighs and hypopnoeas. These events are typically associated with desaturation, and understanding respiratory dynamics that surround such events forms an important part of sleep medicine.
Non-invasive devices typically use bands to detect changes in chest and abdominal wall dimensions to monitor breathing. Such devices are easier to operate technically, and are less disturbing to the subject. However, these devices generally suffer from inaccuracy in measuring or interpreting lung volume changes.
Respiratory inductance plethysmography (RIP) has been used as one such non invasive measure of tidal volume and minute volume in infants but is compromised by complex and time consuming calibration techniques, though a simplified calibration has been recently disclosed. A pneumotachograph and face mask is still required to calibrate RIP to measure volume accurately and therefore can only be used in very young or sedated infants.
A fundamental problem with RIP is the approximation of the infant respiratory system as a two compartment model. In disease states chest wall motion is complex with subcostal and suprastemal recession being typical features. In infants with respiratory distress syndrome, chest wall recession in the inferior aspect of the chest may occur with expansion in the upper portion of the chest, and it is unlikely that a single RIP band can accurately measure thoracic volume changes in such situations. RIP has not been validated as a measure of tidal volume in infants with lung disease, and it is known to be inaccurate in infants under 1.5 kg presumably because of variable chest wall compliance.
Constant volume plethysmography involves the insertion of the infant in a sealed chamber and the application of a face mask to permit the infant to breathe fresh air and to remove expired air. Occlusion of the airway at the mask results in respiratory efforts by the infant for a small number of breaths. This, in turn, compresses and rarefies the gas within the chamber. By measuring the pressure changes, and knowing the volume of the chamber, Boyle""s Law permits an estimate of the total gas within the infant""s lung at the time of occlusion. However, the procedure is technically difficult, and the procedure is not suitable for protracted periods of time, e.g. during sleep.
U.S. patent application Ser. No. 09/124926 describes an improved plethysmograph for measuring infant lung function non invasively during unsedated sleep without the need for a face mask.
It is an object of this invention to provide an improved seal particularly, but not solely, suitable for use with that plethysmograph.
In one broad form, this invention provides a seal suitable for sealing the entry of a neck portion, limb or the like into a chamber, comprising
a pair of juxtaposed flexible sheets each having an opening therein, the openings being aligned, the sheets being sealed together around the openings and along a closed figure around, and spaced from, the openings, to define a closed space between the sheets;
particulate material in the closed space; and
an opening in one of the sheets to permit the closed space to be connected to a vacuum pump or the like, whereby upon evacuation of air from the closed space, the seal adopts a substantially rigid form.
Typically, the flexible sheets are made of elastomeric material, such as latex rubber.
The neck seal is flexible to permit passage of a patient""s head through the openings, but is stiffened in use to a substantially rigid form In this manner, if the neck seal is used in a plethysmograph, it does not substantially alter the constant volume of the plethysmograph chamber during pulmonary monitoring operations.
The particulate material may suitably be particles or beads of expanded polystyrene.
The opening is suitably in the form of a spigot for ease of connection to a tube leading to a vacuum pump.
In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.